The use of plastic patterns which could be burned out of a casting mold has heretofore been known. For example, polystyrene materials have been used to make patterns for casting molds and have been successfully burned out from the molds. Polystyrene has a relatively low melting point of about 165.degree. F. to about 221.degree. F. depending on the grade of material, with minimal expansion and trace impurities. Thus, polystyrene can be successfully used as a pattern to produce castings with dimensional accuracy. When polystyrene materials are heated out in the removal process, the polystyrene expands very little thus putting little pressure on the casting mold. As a consequence, the casting mold retains its shape and parts can be cast therefrom with dimensional accuracy. Further, the low number of impurities in polystyrene results in a burn-out with minimal ash content. Materials which have a high ash content are not desirable since such ash material requires further processing steps to remove or will leave imperfections in the surface of the part casted. In comparison, lithographic materials such as photopolymers can be used to make casting patterns. Unlike polystyrene, these materials expand greatly during the burn-out process which results in an uncontrollable change in the mold cast dimensions and often results in explosion of the casting mold. These types of materials also have associated with them an undesirable high level of ash content upon burn-out.
Polycarbonate has a melting point of approximately 300.degree. F. to about 600.degree. F. and an ash content of which is known to be greater than polystyrene.
Wax materials are commonly used in the making of patterns for the investment casting process. However, these wax patterns have drawbacks. In hot weather, or any warm environment, wax patterns have a tendency to become distorted or even melt. Wax patterns are very fragile and can be easily broken. Further, thin complicated designs are extremely difficult to create in such a pattern due to the weakness of thin wax walls.
Plastic pattern heretofore required expensive injection molding equipment to produce and the injection molding process frequently causes core breakage. Further, it is difficult to consistently position cores in the injection mold. Any plastic pattern must be completely removable by a solvent or have no ash content if the pattern is removed by a burn-out process.
Thus, it would be desirable to provide a process of making pattern which requires minimal handling of a core which the pattern surrounds, a process that does not explode the casting mold and wherein the pattern can be removed to provide a casting mold with dimensional accuracy and little or no ash content.